To fire a crystalline glaze, that glaze must first be applied to something, and that something is, of course, a clay body. Most potters probably never give the composition of the body much thought—except for how it influences the maturing temperature of the clay. And for many types of ceramics, that is an appropriate attitude.

In the next chapter we give you 37 recipes for zinc-crystalline glazes using Ferro 3110. Why so many? Because different glazes have different properties. For instance, some of them fire as low as cone 8, whereas others go as high as cone 12. It is our experience that the higher the temperature, the nicer the glaze. But you may find you prefer the lower-firing ones.

Also, as stated in Chapter 9 on coloring these glazes, a particular colorant will often give quite different colors when added to different base glazes.

Note that zinc-crystalline glazes can be made with a wide variation in the amounts of zinc and silica. Too often we hear that one must use only one precise amount of zinc, for instance, in order to get crystals.

When you get to the point where you would like to develop your own unique base glaze, this large set of recipes will give you a good feeling for the ranges over which you can let your ingredients vary.

Part of the lore of crystalline glazes is that they are influenced by the chemicals in the water they are mixed with. To test this we mixed several batches of glaze with distilled water and a matching number of identical ones with our local tap water.

We found that, at least where we live, it does make some difference. The difference is not large. Firing the glazes 3º to 5ºC (5º to 9ºF) higher with the tap water gives essentially the same results as with the distilled water. We attribute having to raise the firing temperature to the fact that our water is quite hard and contains a significant amount of calcium. Your local water will likely be different, so we suggest you make a similar test. If the distilled water works much better, then that is probably what you will want to use. Cheaper alternatives to distilled water that may also work include de-ionized water and reverse-osmosis water.  Water softeners usually replace calcium with sodium, so if you use soft water, the maturing temperature of your glazes may be reduced by a few degrees compared to using distilled water.

If you don't use a glaze-suspending ingredient in your crystalline glazes, chances are very good that they will separate. That is, after standing for a day or two, they will form a hard layer of glaze in the bottom of the glaze bucket with a thin liquid at the top.

There are several possible glaze-suspension techniques:

• Many use Epsom salts (magnesium sulfate), typically about 1/4% of dry glaze.

• Norkin uses 2% to 3% bentonite plus 0.2% lithium carbonate.

• One of us likes to use a CMC (sodium carboxymethylcellulose) solution.

Why does one of us use CMC and the other not? The answer is that LeRoy uses only rather little glaze at a time (primarily on test tiles) and often wants to store what is left for weeks or even months. Jon, on the other hand, generally uses the entire batch of glaze in one day, since he glazes many large objects. So he uses plain water, with no CMC.

There is no doubt that the "secret" to creating a crystalline glaze is in the firing schedule and that each crystalline glaze must be tested to find its own optimal schedule.

The basic firing schedule is:

1. Heat the kiln up to a high temperature.
2. Possibly hold it there for a while.
3. Lower the temperature.
4. Hold it there for one to several hours.
5. Turn the kiln off.

As simple as the basic schedule is, there are a number of variables you should be aware of. So let's look at that schedule in more detail.

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You will find that if you put three or four identical pots in the same kiln load, each with the same glaze, they very frequently come out looking somewhat different. Why?  Because the heating in the kiln is not exactly uniform. Yes, those small variations in heat can, with some glazes, cause very great differences in crystal sizes, shapes, or colors.

We notice a change in crystal shapes whether we start a firing during the day or during the night!

The major complication in firing crystalline glazes is that at their maximum temperatures they are very fluid and much, if not most, of the glaze will run off the pot during firing. This fluidity is necessary in order for the crystals to form properly.

Three possible techniques for avoiding this flow are:

• Leave a bare area at the bottom of the sides of the pot. When fired, the glaze will run onto this area and (hopefully) not run off the bottom.
• Instead of leaving the area near the bottom bare, apply a standard, noncrystalline glaze there.
• Some say that if the glaze is "properly" formulated, it will not run very much. But those glazes seem to produce only rather small crystals.

We have tried all of these techniques and find that none of them work for us. You might give them a try—they may work for you.

Most crystalline glaziers deal with the flowing glaze by placing the pot on a pedestal inside a catch basin before the firing. After the firing, they break the pot off the pedestal and grind down any sharp edges.

That is not to say that these glazes are never fired in gas kilns. Evans, for instance, fired with gas, but he was very careful to have an oxidizing atmosphere.

In this chapter you will learn how to do both.

Zinc crystalline glazes are certainly the most popular of all crystalline glazes. And for good reason—their often large, intricate crystals can be truly elegant. Although the base (uncolored) glazes, with their white crystals on a clear background are very lovely, the truly spectacular glazes come from adding colorants. Some additives color primarily the crystals, whereas others color primarily the background. And others color both about equally.

Although the high-fire glazes given in Chapter 3 are generally considered to be the most interesting zinc-crystalline glazes, you can form zinc crystals at lower temperatures also  —  at cone 7 and below.

Why would one want to use the low-fire glazes?  Perhaps you are unable (or unwilling) to fire above cone 7, or perhaps you want to expand your range of crystal shapes and colors.

We occasionally use the low-fire glazes because we like the somewhat different effects we get from them. The crystals are usually smaller than in the high-fire glazes and they are frequently of different shapes and colors.

Although much of the appearance of a fired crystalline-glazed piece is "accidental," there are a few things you can do to influence its outcome.

Some people like a few large crystals with lots of the background glaze showing. Others prefer the pot to be completely covered with small crystals. And there are many stages between these two extremes.

The number of crystals may be altered by making changes to either the glaze recipe or the firing schedule (or both).